Improved Electron Injection in Organic Light Emitting Devices by Applying Thin Insulating Layers
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ABSTRACT We present the fabrication and characterization of organic light emitting devices (OLEDs) using thin insulating layers for improved electron injection. The OLEDs are constructed with an ITO anode and an aluminum cathode. For the active layer we use either ladder-type Polyparaphenylene (m-LPPP) or Parahexaphenyl (PHP). A thin film of an insulating material is applied between the active layer and the cathode, in order to achieve a better tunnel injection due to a higher electric field at the interface. We compared different insulating materials with various thickness. The best results are obtained by using a LiF-layer with a thickness between 10 A and 15 A. Thereby the onset voltage decreases and the current density in the device increases significantly. INTRODUCTION After the application of conjugated polymers as organic semiconductors in organic lightemitting devices (OLEDs) had been initially demonstrated [1,2], poly(paraphenylene) (PPP), was the first simple conjugated polymer to produce visible blue light, when used as the active layer [3]. The realization of blue OLEDs is a necessary step for the possible application of the polymers in multicolor screens. To realize OLEDs with operation lifetimes sufficient for industrial application, is much easier with small band gap conjugated polymers, because for these polymers the mismatch between the energy bands of the polymers relative to the electrodes are much smaller than for wide band gap polymer. The charge injection in OLEDs depends exponentially on the energetic barrier height at the interface between the polymer and the electrode. Thus, lower operation voltages are necessary for OLEDs with a small band gap than for high band gap polymers, so that it is easier to realize red-orange and also green OLEDs with long operation lifetimes than it is the case for blue ones. One possibility to decrease the onset voltages (i.e. electric fields) of OLEDs is to insert a thin insulating layer between the active layer and the cathode, as it has been shown recently [4-9]. Suitable insulating layers for this purpose can be made of organic (e.g. Poly(methyl-methacrylate) [4] and some inorganic materials, as Si0 2 and LiF [5-9]. Although this effect has been observed by several groups using various active layers, the fundamental mechanism and the influence of the specific material parameters of the insulating layers onto the device performance are still not clarified [8]. In this article we investigate the influence of various thin insulating layers on the device performance of blue OLEDs based on Parahexaphenyl (PHP), which are produced in a one step UHV-vacuum deposition process, and of OLEDs based on a derivative of ladder-type Polyparaphenylene (m-LPPP) as shown in Figure 1. The device perfomance of the OLEDs is presented and the influence on the elctroluminescence (EL) efficiency is discussed. 69 Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
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C10H21
n C 6H 13
CH 3
0 CloH 21 Figure 1: ChemicalStructures of the used organic material
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